Another data point: The Narita Express is a great way to go if you are familiar with Tokyo, and especially with the metro system and regional train lines. I had light luggage, I would go for the Narita Express in this case.

If you are unfamiliar with Tokyo (or don't remember it that well 'cause it's been a while), and especially if the Limo Buses have a stop right at your hotel, that's a nice way to go to.

In general, the train is (a) faster to
Tokyo Station and (b) probably slower to
many other parts of Tokyo than the bus on
a good traffic day but (c) definitely faster than the bus on a bad traffic day. (I love how trains=no traffic headaches! :-)

-b-